1. Field of the Invention
The present invention generally relates to implantable medical devices, particularly cardiac electrical stimulus generators. More particularly, the invention relates to methods of making hot can stimulation electrodes having a conforming electrically insulative biocompatible coating on the housing. The invention relates still more particularly to methods of selectively removing polymeric coating material from such devices to expose a conductive area of the housing.
2. Description of the Related Art
Biocompatible coatings for implantable medical devices are widely employed to avoid adverse body responses to the implanted foreign object. In many modern implantable heart stimulus generators, a biocompatible coating that adheres to and conforms closely about the generator housing (hereinafter "a conformal coating") also serves as an electrically insulative layer covering the generator housing.
In the infancy of implantable cardiac pacemaker therapy, the typical pulse generator case or housing was composed of uncoated metal such as stainless steel, titanium or alloys thereof, and was therefore completely electrically conductive. While this permitted the generator case, or housing, to serve as one of the stimulus electrodes, it also allowed an antenna effect between the generator case and the endocardial electrode tip to occur. When flexation of the pectoral muscle occurred, voltages of similar amplitude and frequency to the intrinsic cardiac signals were produced and conducted through the generator case. Spurious undesirable and potentially dangerous influences on the pacemaker functions resulted, the effects including improper inhibition, and in the dual chamber pacemakers, improper triggering and initialization of reentry tachycardias. In addition to the antenna effects on the pacemaker, each time the pacer emitted an electrical pulse to stimulate the myocardial tissue it also stimulated the patient's pectoral muscle, producing an annoying twitch.
The next generation of pulse generators avoided the muscle-produced improper triggering of the unit by coating the entire electrically conductive case with an electrically insulating material, except for a small uncoated window that allowed the exposed housing surface to serve as the anodal electrode contact. A coating material commonly used for this purpose on pulse generators is typically a thermoplastic polymer film known commercially as parylene, which is both biocompatible and an excellent electrical insulator. In a typical parylene coated unit, the posterior side of the case, meaning all parts facing the (inside) pectoral muscle, all side walls, and part of the anterior (frontal) side of the case are coated with parylene, leaving only a small part of the anterior side of the case to form a forward-facing anodal window that faces the (outside) fatty tissue. This orientation of a "face window" type pulse generator unit greatly reduces muscle-induced interference and has been clinically proven and implemented in many thousands of implantable pacemakers and defibrillators.
Recently, Sulzer Intermedics Inc. provided in U.S. Pat. No. 5,480,416 a cardiac stimulator having its anterior (front) and posterior (back) sides of the case coated with an electrically insulative material such as parylene, but having the edge connecting those two sides at least partially uncoated. Thus, the edge or narrow side of the case functions as the electrically conductive anodal contact surface. This conductive "edge band window" configuration allows for universal implantation orientation of the unit, permitting the pulse generator to be implanted in either the "normal" position conventionally dictated by the placement of the outlets for the connector and lead, or just as conveniently, in the reverse orientation on the opposite side of the body. In this way, the generator case can be turned for left exit or right exit of the electrode lead, and can be implanted in the left or the right side of the patient's chest, at the option of the physician. This edge band window approach also avoids the muscle-induced interference and inappropriate muscle stimulation problems encountered with prior "hot" or "active can" electrodes.
Sulzer Intermedics, Inc. has also developed an implantable defibrillator with the conformal coating partially removed to expose conductive windows (U.S. Pat. No. 5,529,579, issued to Alt et al.).
The window in a conventional parylene coated face window pulse generator is typically formed by an oxygen plasma etch process. The plasma etching process usually employs an aluminum fixture or stencil to define the shape and location of the parylene window on the face of the can. In this process, oxygen is ionized by an RF power supply generated electric field. The ionized oxygen gas cloud, or plasma, reacts with the organic parylene in the window region and forms carbon dioxide gas and water vapor. These gases are subsequently removed by a vacuum. The etch rate is controlled by the RF power and the heat generated at that power. High frequency radio frequency power creates a more reactive plasma than does low frequency power. Higher temperature creates a more reactive plasma, as well. The inherent limitations of the battery inside the pulse generator require that the temperature of the entire unit remain below 60.degree. C. during the etching process, however. This consideration severely limits the rate at which cans can be passed through the window-etching process. Also, it is necessary to maintain electrostatic shielding to prevent undesirable plasma reactions with the vacuum chamber internal surfaces.
Alternatively, the window can be formed simultaneously with the application of the parylene coating, using a masking technique. For the purposes of a human clinical study, for example, Sulzer Intermedics Inc.'s Edge Band units were manufactured by masking the anodal edge region with weld shield tape, parylene coating the entire unit, then removing the tape. This procedure is time consuming, however, and not feasible for large scale production.
U.S. Pat. No. 5,562,715, issued to Czura et al., describes a silicone rubber or parylene coated pacemaker having detachable tabs that are removed at the time of implantation to expose an electrode. One problem with pacemakers employing pull off tabs is that the resulting window in the coating material has jagged or rough edges that leave the remaining coating vulnerable to tearing, peeling or flaking off into the patient's body. A rough-edged parylene coating is especially prone to peeling, tearing or flaking while the unit is being manipulated during surgical implantation and thus allows dislodgment of flecks of the coating into the patient's body. Parylene C is an organic polymer based on p-xylylene, ##STR1## having a molecular weight of about 500,000 daltons. It is used as a thin film coating for such diverse applications as microelectronics, digital display systems, medical devices, dry film lubricants and reflectors for optical devices..sup.6 Some of the methods that have been used or examined for removing parylene from various surfaces include heating, mechanical/air abrasion, plasma etching, and excimer laser ablation..sup.1,4
For cardiac stimulus generators, however, methods that require heating are not suitable because the melt temperature of most polymers is far greater than the maximum 60.degree. C. exposure limit imposed by pacemaker battery manufacturers. Mechanical or air abrasion can create static discharges and make edge and face definition virtually impossible. Plasma etching is too slow, requiring over 10 hours to remove the coating layer from the face of a single pulse generator unit. Because the rate of plasma etching is dependent on temperature, achieving faster results would require exposing the unit to temperatures much higher than the internal components can tolerate. Nevertheless, plasma etching remains the prevailing standard method in the pacemaker industry.
Parylene, and other similar biocompatible coatings, also find widespread application in coating medical devices other than implantable electrical pulse generators. Some of these devices are sensors, probes, transducers, stimulators and prostheses..sup.7 In many of these devices it may be desirable to provide a window or uncoated portion similar to the pacemaker window described above.
Recently, coatings for medical articles that incorporate functionally active biomolecules capable of eliciting a particular desired effect in the body have been described. For example, U.S. Pat. No. 5,607,475, assigned to Medtronic, Inc., discusses attaching such biofunctional molecules as anticoagulants, thrombolytic agents, cell attachment proteins and anti-inflammatories to coated surfaces of medical devices by way of a covalent bond. It is anticipated that portions of these coatings will likewise need to be removed in some instances.
Hence what is needed is a more efficient way to manufacture high quality implantable medical devices having biocompatible or biofunctional coatings. In particular, a precision method for selectively removing such coatings from certain areas of the device or article is desired.